Abstract
Hydroxide films were formed on aluminum specimens by immersing them in water at temperature (Tb) of 100-180°C, and the specimens were then anodized galvanostatically in a neutral borate solution to form composite oxide films. The formation behavior of the hydroxide and composite oxide films were followed by gravimetry, X-ray photoelectron spectroscopy, electron microscopy, and electric capacitance measurements.
It was found that the growth rate of the hydroxide films increased with increasing Tb, and that the hydroxide/metal interface roughened increasingly due to the non-uniform growth of the hydroxide. The water content of the hydroxide (value of X in Al2O3·XH2O) decreased from 2.7 to 1.8 when Tb increased from 100 to 180°C.
The composite oxide films that formed after boiling at different values of TbS were all composed of an outer crystalline oxide layer (thickness δo) and an inner amorphous oxide layer (thickness δi), and while δi did not depend on Tb, δo decreased with increasing Tb. The composite oxide films formed by anodizing to Ea=300V showed thickness/voltage ratios ((δo+δi)/Ea) of only 1.10 to 0.87nm/V, decreasing with increasing Tb. The capacitance and dielectric loss of the composite oxide films increased with Tb.
The effect of Tb on the formation behavior of composite oxide films is discussed by considering the volume changes caused by the field-assisted dehydration of hydroxide to form a barrier oxide layer.
